The present application relates to lift cranes, and particularly to high-capacity mobile lift cranes having a mast behind the main boom from which counterweight is supported, and to methods of raising that mast during a set-up operation.
High-capacity mobile lift cranes typically include a carbody having moveable ground engaging members; a rotating bed rotatably connected to the carbody such that the rotating bed can swing with respect to the ground engaging members; a boom pivotally mounted on a front portion of the rotating bed, with a hoist line extending therefrom; a mast mounted on the rotating bed; and counterweight to help balance the crane when the crane lifts a load. The mast is used to support the rigging, including the boom hoist rigging, so that it may transfer the forces from lifting a load to the rear of the carbody and the counterweight. Sometimes an extra counterweight attachment, such as a counterweight trailer, is added to the crane to further enhance the lift capacity of the mobile lift crane. Since the load is often moved in and out with respect to the center of rotation of the crane, and thus generates different moments throughout a crane pick, move and set operation, it is advantageous if the counterweight, including any extra counterweight attachments, can also be moved forward and backward with respect to the center of rotation of the crane. In this way a smaller amount of counterweight can be utilized than would be necessary if the counterweight had to be kept at a fixed distance. The mast has to be designed to support the rigging, including a tension member, such as a counterweight strap, tied to such counterweights. The mast for such high-capacity cranes is usually made from a plurality of lattice segments and is designed to withstand very substantial compressive loads.
Since the crane will be used in various locations, it needs to be designed so that it can be transported from one job site to the next. This usually requires that the crane be dismantled into components that are of a size and weight that they can be transported by truck within highway transportation limits. Thus, any weight reductions that can be achieved with the design of the crane help with not only its initial cost, but with the cost of transporting it between jobs for the life of the crane. Further, the ease with which the crane can be dismantled and set up, and the need for assist cranes to do that, has an impact on the total cost of using the crane. Thus, to the extent that fewer man-hours are needed to set up the crane, including the man-hours needed to set up an assist crane, there is a direct advantage to the crane owner.
Most high-capacity cranes are set up with the mast, which is first assembled from segments on the ground, being lifted by an assist crane and attached to the rotating bed. Typically the mast is positioned so that it extends out over the front of the crane. Rigging connected to a gantry on the rear of the crane is then used to pull the mast into an upright position. Since it will eventually be positioned so as to lean backwards, it has to be pulled over a vertical position. Of course at this point the mast will start to fall. Thus there must be tension applied to the mast from the front as it passes over center to prevent it from falling. This is usually provided by a hold-back assist crane, or the boom hoist rigging is installed and attached to the boom to provide a counter force. When the boom hoist rigging is used, the crane set-up operator has to be very skillful so as to draw rope on the mast hoist spools while simultaneously paying out rope for the boom hoist rigging so as to controllably bring the mast to its working position.
Some high-capacity cranes are even more complicated, such as the Liebherr LR11350, using a derrick mast as well as a moving machine mast, the derrick mast being moveable as the extra counterweight unit is moved in and out. This derrick mast is assembled in the same way the main mast described above is added to the crane, requiring the derrick mast to be pulled over top center from its initial over-the-front position.
In addition to the fact that the lift enhancing mast is installed and raised from an over-the-front position, requiring highly skilled crane operators in the set up of the crane, the structure needed to raise the mast is also substantial. The cranes need a structure, either a gantry or moving mast, to provide a moment arm about the mast hinge pin. There must also be a powered drum, rope, associated sheaves, multipart reeving and mast raising hardware. As mentioned above, there must also be a means to hold the mast back as it approaches the over-center position and then is controllably brought to its working position.
U.S. Pat. No. 4,349,115 to Lampson discloses a crane that has a mobile counterweight unit separate from the main crane carbody. A mast is used on this crane as with other high-capacity mobile cranes. The Lampson patent discloses a set-up operation wherein the mast is first attached to the rotating bed and extends backwards over the rear of the crane. The mast is attached at its outer end to a counterweight strut, which in turn is attached to the mobile counterweight unit. These are assembled near ground level. An assist crane is used to raise the connection at the mast and the counterweight strut to near its working height, at which time the counterweight unit can be brought close enough to the rotating bed that a spreader link can be connected between the rotating bed and the mobile counterweight unit. While this set-up operation avoids the need for a gantry, powered drum, sheaves and reeving, it still requires an assist crane that has the capacity to raise not only the mast, but the counterweight strut as well, and to lift them up to a height where the boom hoist rigging and weight of the boom can act as a counterweight to pull the mast up to its final working position. The assist crane must have a fairly long boom to accomplish this.
Thus there is a need for further improvements in high-capacity mobile lift cranes, particularly in the structure and procedure used to raise the mast.